Scanning of sawn lumber is required in order to assess the severity and location of knots, checks and other surface defects in each board passing through a mill. This information determines the grade of each board, and can be used to assess the desirability of trimming the board to increase its grade. Typically, a board is trimmed in two foot increments. The grader must assess whether and how much is to be trimmed from either end of a board, weighing the loss of lumber against the upgrading that can be achieved by removing a portion of the board that bears surface defects. The grade given each board is a function of grading rules determined by an appropriate governing body, and is determined by the presence of surface features such as knots, checks, and the like, their location in the board, e.g., adjacent an end or an edge of the board, and the size, density and general severity of the features.
The grading of lumber is typically performed manually. A mill worker typically picks up one end of a board and scans it for surface features by gazing along its length. The worker estimates the amount that is required (or desirable) to be trimmed from each end of the board, and assesses the resulting lumber grade. A relatively high degree of error occurs in the manual scanning process, from the difficulty of gauging accurately the position of a defect when looking along the length of a board that may be up to 24 feet long, the typical maximum length board processed in a mill. In an attempt to minimize error, lumber graders generally overestimate the amount that is required to be trimmed, especially from the far end of the board, and a significant portion of the lumber is thus wasted. Automation of the grading task is desirable, both to economize on manpower and to minimize error in the scanning process.
It is known to employ optical scanning means for lumber processing for purposes other than grading and trimming. For example, U.S. Pat. No. 4,803,371 (Durland) discloses an optical scanner, intended to resolve the three-dimensional configuration of a rough sawn timber passing through the device, in order to assess the optimal cutting of the timber into finished or semi-finished boards. The device employs a plurality of light sources positioned to illuminate each board passing through the device, and a number of image detectors having overlapping fields of view, positioned above and below the board, adapted to resolv the three dimensional configuration of the board. The device may also be used to assess surface defects on the board. It has heretofore been difficult to achieve automatic scanning of lumber for grading and trimming purposes. It is necessary that all four longitudinal faces of each board be scanned, and this requires either the positioning of scanning devices above and below the level of the boards, or means for rotating each board. The former approach suffers the drawbacks of debris accumulation on the lower scanning devices, and obstruction of the lower scanning devices by the various beams and girders required to support the lumber. The latter approach, selected in the present invention, requires the provision of a simple, rapid and effective means to rotate each board in order to expose all four of its longitudinal faces, and this has not heretofore been achieved. An automatic scanner must achieve accurate, reliable results at the high speeds required in a modern mill. It is desirable to provide a modular system, whereby each scanning device independently scans a segment of the board, in order to allow for a variety of board lengths to be scanned, and to facilitate repairs to the device.